Recent advances in genomics, including SAGE and DNA microarrays, have allowed researchers to perform high throughput analysis of gene expression. These experiments generate large amounts of information, which must be confirmed by independent techniques one gene at a time. Cellular activity and function are controlled by protein activity, which cannot be easily predicted from measurement of steady state mRNA levels. Under some conditions, for example, increases in mRNA levels do translate to increased protein abundance. However, under some stresses, elevated mRNA levels are required to keep protein levels constant (Ideker et al., Science 292:929-934, 2001).
New techniques are being used to study cellular protein expression and function. For example, tissue arrays constructed to hold hundreds of tissue profiles from normal and diseased tissues can be sectioned, with each section being used to evaluate a different disease marker by immunohistochemical staining. This technique requires solid tissue samples, and antibodies that bind to formalin-fixed paraffin-embedded samples. Also, the target features of the tissue must be adequately represented within a 100-300 micron spot. The cellular heterogeneity of the kidney makes if difficult to ensure that even one glomerulus is contained within each sample.
Many techniques have been developed to study protein expression; however, they require expensive equipment (2-D mass spectroscopy, Ideker et al., Science 292:929-934, 2001; Ciphergen ProteinChip; Weinberger et al., Pharmacogen. 1:395-416, 2000; protein arrays, Paweletz et al., Oncogene 20:1981-1989, 2001, for instance), or prolonged or proprietary chemistry to attach proteins to solid support (for instance, with protein chips, see Zhu et al., Nat Genet. 26:283-9, 2000). Inexpensive hand stamping methods have been developed and are available commercially; however, only one or two arrays can be made at a time.